1) Look
at the primary coverts - on HY/SY birds they are
paler than greater coverts with little or no edging,
and on AHY/ASY birds they are uniform with the
greater coverts and have distinct blue edging

2) Look at
the tail - on HY/SY birds the rectrices are
fairly narrow and pointed, and on AHY/ASY birds they
are relatively broad and rounded at the tip

3) Look
at the overall plumage - within each age class
males tend to be more bluish with larger white
patches on the rectrices, while females are more
greenish and have smaller white patches on the
rectrices

After-second-year females are somewhat paler
than males of the same age and lacking a chestnut breast band. However, they may be similar in overall appearance to
hatch-year males; the wing and tail should be examined to confirm
identification. The blue edging on the primary coverts may be
visible on a perched bird, as in the second photo below, and this can be used
to identify it as an after-second-year individual, as second-year birds
lack this edging and have noticeably more worn and brownish primary
coverts.

Photo by Marie-Anne Hudson,
McGill Bird Observatory (QC), May 2007

Photo by Barbara Frei,
McGill Bird Observatory (QC), May 2008

Note the overall bluish tone to the wing, and especially the bluish
edging to the broad and rounded primary coverts.

Photo by Marie-Anne Hudson,
McGill Bird Observatory (QC), May 2007

Photo by Barbara Frei,
McGill Bird Observatory (QC), May 2008

The rectrices are broad and fairly rounded, with a moderate amount of
white on the outer two feathers, and typically most to all rectrices are
in good condition.

Second-year
males are somewhat paler and more greenish than older males, but similar
in overall appearance to after-hatch-year females; the wing and tail should be examined to
confirm identification. Often the brownish and somewhat worn
primary coverts are visible on a perched bird, and are a strong
indicator that it is second-year.

Photo by Marie-Anne Hudson,
McGill Bird Observatory (QC), May 2007

Note the relatively dull and worn primary coverts, uniform in colour and
condition with the primaries and secondaries, but contrasting with the
fresher and bluer greater coverts.

Photo by Marie-Anne Hudson,
McGill Bird Observatory (QC), May 2007

The rectrices are typically somewhat narrow, tapered, and worn; the
example below appears particularly tattered as a result of being a bit
wet.

After-hatch-year
males are the most boldly coloured, with upperparts mostly blue,
contrasting with the distinct greenish back triangle sometimes tinged
with orange, and bold yellow underparts, also sometimes tinged with
orange or chestnut but often not.

The wing is particularly useful for ageing and
sexing this species. The wing shown below is typical for an
after-hatch-year male, with distinct blue edging on primary coverts, and
bluish edging also on the primaries and secondaries.

After-hatch-year males
have the greatest amount of white on the tail, often extending to r4;
all other ages and sexes have white limited to the two outermost
rectrices. The rectrices also tend to be quite broad and rounded.

After-hatch-year females are somewhat paler
than males of the same age, but similar in overall appearance to
hatch-year males; the wing and tail should be examined to confirm
identification. Note though that the blue edging on the primary
coverts, indicative of an after-hatch-year Northern Parula, can often be
seen while the bird is perched, as in the photo below.

Hatch-year
males are somewhat paler and more greenish than older males, but similar
in overall appearance to after-hatch-year females; the wing and tail should be examined to
confirm identification. Note though that the lack of blue edging
on the primary coverts, indicative of a hatch-year Northern Parula, can
sometimes be seen on a perched bird

The edging on the primary coverts is less distinct
than on after-hatch-year birds, but more noticeably, the primary coverts
are significantly paler than the blackish greater coverts, and the
edging on the primaries and secondaries is greenish.

Tail shape is not the
most reliable feature for Northern Parulas, but hatch-year birds do tend
to have slightly narrower rectrices that taper to more of a point.
White patches are limited to the outer two rectrices.

The edging on the primary coverts is less distinct
than on after-hatch-year birds, but more noticeably, the primary coverts
are significantly paler than the blackish greater coverts, and the
edging on the primaries and secondaries is usually more greenish than blue.

Tail shape is not the
most reliable feature for Northern Parulas, but hatch-year birds do tend
to have slightly narrower rectrices that taper to more of a point, and
on some individuals such as the one below where that shape is
pronounced, it can be a strong indicator of age. White patches are
limited to the outer two rectrices, and the spot on r5 is often
particularly small.